Gamma Ray Spectroscopy 1 GAMMA RAY SPECTROSCOPY

In this experiment you will use a sodium iodide (NaI) detector along with a multichannel analyzer (MCA) to measure gamma ray energies from energy level transitions within the daughter nuclei of three low-level radioactive isotopes. Theory Only certain discrete energy levels are possible in an atomic nucleus, similar to the situation with atomic (orbital) electrons. When the nucleus makes a downward transition from an excited state, a very high energy, very short wavelength photon called a gamma ray may be emitted. The energies of these gamma rays are determined by the spacing of the allowed energies within the nucleus, just as the characteristic visible, ultraviolet, and X-ray spectrum of an atom is determined by the allowed energies of the electrons around the nucleus. However, since the nuclear force is much stronger than the Coulomb force, the energy transitions and photon energies for nuclear decay are much larger (typically MeVs) than the energies for visible (eV) or X-ray (keV) photons. For this experiment, the SpecTech will again be used. However, this time the channels will not correspond to time intervals; rather, in the MCA mode, the channels correspond to different energies. The MCA reads the magnitude of an input electrical pulse and assigns the event to the "channel" which represents the energy associated with the pulse. In this way, the MCA keeps track of the number of events occurring at each value of energy. The detector used in this experiment employs a thallium-doped sodium-iodide crystal. A gamma ray striking this crystal may transfer all of its energy to an electron in a process called a Photoelectric Interaction. The electron recoils, knocking off other electrons which results in the emission of a tiny bit of light. This light is converted to an electric pulse. Since the amount of light depends on the energy given to the electron by the gamma, the size of the electric pulse is related to the energy of the gamma. And since every gamma from the same transition has the same energy, the MCA - which displays the number of pulses of each energy - will therefore display a peak in the channels that correspond to that energy. Such peaks are shown in Figure 1. It is also possible for a gamma to give less than all of its energy to an electron. This results in a lot of pulses of in the lower energy channels of the MCA display. We will not be bothered with those in this class. Rather, we will concentrate on the peaks, which give the actual gamma energies. Experiment Part 1: Adjust and Calibrate the MCA

1. Open the “USX” program. Click on MODE and assure that “Pulse Height” is selected.

2. Assure that the high voltage is ON and set to the voltage labeled on your NaI tube.

3. The SETTINGS: Amp/HV/ADC menu also controls the resolution of the analysis by

Gamma Ray Spectroscopy 2 setting the number of channels (bins) into which the counts are put. We will use 1024. There are also discriminator settings. Set the LLD to 13.

4. Above the plot window are the amplifier controls, which spread the various pulse heights across the channels. The amplifier has both coarse gain (powers of 2) and a fine gain control. Since the highest energy peak of interest in this experiment is the 1.33 MeV peak from 60Co, use it to set the amplifier gains. Put the 60Co source in front of your detector. Set the gains so that both high energy peaks are on the far right side of the display. (See Figure 1.)

Figure 1. The twin photo peaks from 60Co

5. Now we begin the calibration. Put the cursor on the highest energy 60Co peak (farthest to the right, of course). Record the channel number. Then clear the display.

6. Switch to the 137Cs source. Make sure the X-ray peak (shown in Figure 2) stands out.

Put the cursor on the X-ray peak and record the channel number. Also, put the cursor on the peak that is in the middle of the screen and record its channel number.

7. Click on SETTINGS: Energy Calibrate/3 Point. The software will ask for three points,

each one requiring the channel number and associated energy for the peaks you recorded. Use the 137Cs X-ray peak for one, with energy = 32 (KeV). For the second point, use the other 137Cs peak with energy 662 KeV. For the third point, use the high energy 60Co peak channel you found in Step 5 and for its energy use 1333 (KeV).

Gamma Ray Spectroscopy 3

Figure 2. The 137Cs Spectrum Part 2: 137Cs spectrum

Take a 137Cs spectrum and print it out. Move the on-graph cursor to the top of each peak and find the energy of the gammas that produced that peak. On the print-out, identify the gamma ray peak and the x-ray peak shown in Figure 1. Write your measurement of the energies above the peaks.

Finally, find the gamma ray transition in the appropriate energy level diagram hand-out. Highlight the transition and include that copy of the energy level diagram with your deliverables. Part 3: 60Co Spectrum In 60Co, two different gamma rays are emitted during every decay into 60Ni; thus its gamma ray spectrum will show two photo peaks. Take a 60Co spectrum. Identify two photo peaks and write their energies above them on the print-out. Find the two gamma transitions in the energy level diagram.

Gamma Ray Spectroscopy 4 Part 4: 22Na Spectrum Certain nuclei, such as 22Na, decay by emitting a fast positron rather than an electron (negative beta particle). The positron is the anti-matter version of the electron and is identical to the electron in all respects, except that it carries a positive charge of +e. When the positron slows and encounters an electron in the plastic sample disk, the two "annihilate" one another, meaning that all their rest mass is converted into pure energy: the electron and positron vanish leaving two gamma rays that go off in opposite directions. By measuring the energy of these annihilation gammas and knowing the mass of an electron, you can use E = mc2 to confirm Einstein’s most famous discovery. (Re: the pre-lab questions.) In addition, the 22Na daughter produces a gamma ray. Thus, the decay of 22Na results in two peaks. Take the 22Na spectrum. Identify the two peaks and their energies as above. Record the rest energy of the electron from your results. Find the beta and gamma transitions in the energy level diagram. To turn in:

– copies of spectra for 137Cs, 22Na, 60Co with photo peak energies listed – copies of the associated energy level diagrams with transitions highlighted – summary and discussion of your qualitative and quantitative findings

Pre-lab Questions

1. In the β+ decay of 22Na, what is the daughter nucleus? In the periodic table of the elements, does the daughter come before or after 22Na? Contrast this with the β- decay of 137Cs.

2. Using the appropriate energy level diagram, find the energy level of 137Ba that results

in the 0.662 MeV gamma ray seen in the decay of 137Cs. What percentage of the β- decays lead to that level?

3. What is the rest energy of an electron in MeV? Cite your source of information.

4. What is the mass of an electron in kilograms? Cite your source of information.

5. Using 3.0x108 meters/second as the speed of light, calculate the rest energy of an

electron in Joules. (See Part 4, above. If mass is in kilograms, and speed is in m/s, the result will be energy in Joules.)

6. Finally, knowing that 1 MeV = 1.6x10-13 Joule, confirm your answer to Question 3.

Gamma Ray Spectroscopy 5 Energy Level Diagrams

Gamma Ray Spectroscopy 6